Previous work on this project found significant racial differences in calcium retention and kinetics in adolescent girls. Black girls had greater calcium retention than white girls (461 vs. 293 mg/d) as a result of greater net bone formation rates, increased calcium absorption and decreased urinary calcium. These racial differences in calcium retention can account for the approximately 10 percent greater bone mass in adult black vs. white women. From these studies, we developed a nonlinear regression model of the relationship between calcium intake and calcium retention that predicts calcium requirements for maximal skeletal accretion. Caucasian boys retain more calcium than girls independent of intake, but the intake for maximal retention is the same for both genders. We now propose to study these same relationships in Chinese American adolescent girls and boys. We hypothesize that calcium requirements to maximize bone health will be lower for Asians than Caucasians, but higher than their current intakes. Our primary aim establishes calcium requirements in Asian girls and boys. Calcium absorption efficiency is thought to be higher in Asians than Caucasians despite reduced serum parathyroid hormone concentrations. Our second aim is to establish the relationship between calcium load and fractional calcium absorption between Caucasians vs. Asian adolescents and the mechanisms responsible for racial differences. Our third aim is to determine the relative contribution of the intestine, kidney, and bone to calcium retention in adolescents and to compare racial differences. Calcium retention will be determined over a range of calcium intakes between 600-2300 mg/d during three-week metabolic studies. Stable isotope kinetic analysis will allow us to determine the relationship between calcium intake and calcium absorption and bone turnover rates in Asians. Serum profiles of parathyroid hormone and 1,25-dihydroxyvitamin D following low and high calcium intakes in Asian and Caucasian adolescents will be used to determine racial differences in response to diet and sensitivity of calcium absorption efficiency to these regulators. Optimizing calcium retention during periods of rapid growth is a key nutritional strategy to build peak bone mass and reduce lifelong risk of osteoporosis.